Update - Added the translation of the original article, thanks to Treeston!

Patch 4.3 - Deathwing, Transmogrifier, Void Storage
Tom Chilton (Kalgan) answered a couple of questions from Gamona.de on Patch 4.3, and we got a bunch of very interesting news out of it. Patch 4.3 will feature:

New Raid - The Deathwing raid, making it very likely to be the last patch before the next expansion.

3 new 5-man instances - Including one in the Caverns of Time. (War of the Ancients was mentioned as a cataclysm instance a few years ago, but a lot of things have changed since then, who knows!)

The Transmogrifier NPC, which will let you change the appearance of any armor piece on you, while retaining the stats of your current gear. (= T12 stats, but with the Tier 2 look!)

Players won't be able to change the appearance of their armor into another armor type or class armor set, and you might have to own the armor before you can use it to swap the appearance of your existing armor.

The Void Storage is a new type of bank where players will be able to store their old armors in order to free up space in their regular bank.

The Abyssal Maw dungeon is gone for the moment, the storyline wasn't convincing enough/fitting.

If someone feels like adding information or even translating the article completely, post in comments!

Translation of the original article on gamona.de
We're at GamesCon in Cologne, Germany, and just did an interview with Tom Chilton, Lead Game Developer and Chris Robinson, Lead Art Director, in which they revealed a lot of details regarding patch 4.3 - and it seems that the announcement of it being huge was no joke:

Patch 4.3 will bring us the Deathwing raid instance! This means it's likely that we'll be able to fight the Aspect of Death in an epic encounter within this year. The raid that was originally expected for this patch, the "Abyssal Maw", was cut from the schedule because, as Chilton states, the setting and the story were not convincing enough.

Furthermore, 4.3 will also bring three entirely new 5-man instances - Chilton assured us that these won't contain any re-vamped content, but instead be brand new with unique design and models. Chilton also hinted at one of those being within the Caverns of Time, allowing players to experience another great moment of Warcraft Lore first-hand.

The interview also revealed the purpose of the "mystery ethereals" in Stormwind (and likely other cities, too). The "Transmogrifier" NPC will allow players to change the displayed model of equipment to that of another item while keeping the original attributes: Transmogrification is only possible within an armor class - Plate armor can only assume the looks of other Plate armor, for example.

The developers are considering whether they should only allow using the models of equipment that has already been obtained

The "Void Storage" is a very large bank-esque storage which allows old equipment to be stored, freeing up inventory/bank slots for players. The developers aren't sure yet about the definition of "old equipment". The cost of the storage (and whether there even is one) is another thing that is not yet decided

Another possible feature that was hinted at is the possibility of "void stored" equipment being available to other characters on the account (NOTE: That part might be speculation from Gamona, not cross-server transfers actually confirmed. Hard to tell.)

Overall, it seems that we've got a huge patch ahead of us - a new raid, three brand new instances, Transmogrification and Void Storage.

Epic. Looking forward to finishing off my T12 set, then I can go back and farm up some old bits to match ;DWonder if weapons will be included? Specifics, dammit, people! We need data! Can't theorise without data! Happy player, signing off.

I can believe Deathwing will be in 4.3, but I don't think it will be the last patch. Maybe we'll face N'zoth as the last tier in Cata, just like in TBC, Illidan wasn't the last bad guy, even being the "main villain" for so long. Also, can't wait for the new NPC. More than half of my bank is old gear.

I'm not going to lie. I'm extremely disappointed in the fact that WotA isn't going to be a raid. The concept behind it, along with the number of true lore-based boss possibilities, was making me giddy.

Well, the good thing about void storage is I now can put all those armor sets I've kept like the old ZG rep armor and the Scourge invasion sets. The bad thing is I was hoping for an account-wide bank as a way to transfer BoAs.

I think that we will see deathwing but have my doubts about being the final encounter... A little like in ICC 5mans. We saw arthas but it wasn't the final patch. At least I hope. We will probably fight deathwing in War of the Ancients or something like that. It features deathwing but it wont the the deathwing raid.